Particle Image Velocimetry Part - 1

Transcription

1 AerE 545X class notes #23 Particle Image Velocimetry Part - 1 Hui Hu Department of Aerospace Engineering, Iowa State University Ames, Iowa 50011, U.S.A

2 Announcement Room 1058, Sweeney Hall for Lab#4 (LDV laboratory) Sweeney Hall is where Chemical and Biological Engineering Department is located.

3 Particle-based Flow Diagnostic Techniques Seeded the flow with small particles (~ µm in size) Assumption: the particle tracers move with the same velocity as local flow velocity! Flow velocity V f = Particle velocity V p Measurement of particle velocity

4 Frequency-shift based methods Frequency-shift methods: Based on the Doppler phenomenon, namely the shift of the frequency of waves scattered by moving particles. Laser Doppler Velocimetry(LDV) ) or Laser Doppler Anemometry (LDV) Planar Doppler Velocimetry (PDV) or Planar Doppler Anemometry (PDA)

5 Particle Image Velocimetry (PIV) technique Particle displacement method: to measure the displacements of the tracer particles seeded in the flow in a fixed time interval. ΔL t= t 0 +Δt t=t 0 ΔL U = Δ t a. T=t0 b. T=t0+4ms Corresponding Velocity field

6 PIV System Setup Particle tracers: Illumination system: Camera: Synchronizer: Host computer: to track the fluid movement. to illuminate the flow field in the interest region. to capture the images of the particle tracers. the control the timing of the laser illumination and camera acquisition. to store the particle images and conduct image processing. seed flow with tracer particles Illumination system (Laser and optics) camera Synchronizer Host computer

7 Tracer Particles for PIV Tracer particles should be neutrally buoyant and small enough to follow the flow perfectly. Tracer particles should be big enough to scatter the illumination lights efficiently. The scattering efficiency of trace particles also strongly depends on the ratio of the refractive index of the particles to that of the fluid. For example: the refractive index of water is considerably larger than that of air. The scattering of particles in air is at least one order of magnitude more efficient than particles of the same size in water. hν Incident light Scattering light. d=1μm b. d=10μm c. d=30μm

8 Tracer Particles for PIV A primary source of measurement error is the influence of gravitational forces when the density of the tracer particles is different to the density of work fluid. 2 ( ρ p ρ) U g = d p g 18μ The velocity lag of a particle in a continuously acceleration fluid will be: r r r 2 ( ρ p ρ) U s = U P U = d p g 18μ r r t U p ( t) = U (1 exp( ); τ s 2 ρ p τ s = d p 18μ U r U r P

9 Tracers for PIV measurements in liquids (water): Polymer particles (d=10~100 μm, density = 1.03 ~ 1.05 kg/cm 3 ) Silver-covered hollow glass beams (d =1 ~10 μm, density = 1.03 ~ 1.05 kg/cm3) Fluorescent particle for micro flow (d=200~1000 nm, density = 1.03 ~ 1.05 kg/cm3). Quantum dots (d= 2 ~ 10 nm) Tracer Particles for PIV Tracers for PIV measurements in gaseous flows: Smoke Droplets, mist, vapor Condensations. Hollow silica particles (0.5 ~ 2 μm in diameter and 0.2 g/cm3 in density for PIV measurements in combustion applications. Nanoparticles of combustion products

10 illumination system The illumination system of PIV is always composed of light source e and optics. Lasers: : such as Argon-ion laser and Nd:YAG Laser, are widely used as light source in PIV systems due to their ability to emit monochromatic light with high energy density which can easily be bundled into thin light sheet for illuminating ing and recording the tracer particles without chromatic aberrations. Optics: : always consisted by a set of cylindrical lenses and mirrors to shape the light source beam into a planar sheet to illuminate the flow field. Laser beam Laser sheet laser optics

11 Double-pulsed Nd:Yag Laser for PIV

12 Optics for PIV

13 Cameras The widely used cameras for PIV: Photographic film-based cameras or Charged-Coupled Coupled Device (CCD) cameras. Advantages of CCD cameras: It is fully digitized Various digital techniques can be implemented for PIV image processing. Conventional auto- or cross- correlation techniques combined with special framing techniques can be used to measure higher velocities. Disadvantages of CCD cameras: Low temporal resolution (defined by the video framing rate): Low spatial resolution:

14 Interlaced Cameras The fastest response time of human being for images is about ~ 15Hz Video format: PAL (Phase Alternating Line ) format with frame rate of f=25hz (sometimes in 50Hz). Used by U.K., Germany, Spain, Portugal, Italy, China, India, most of Africa, and the Middle East NTSC format: established by National Television Standards Committee (NTSC) with frame rate of f=30hz. Used by U.S., Canada, Mexico, some parts of Central and South Old field America, Japan, Taiwan, and Korea. 1 frame (1,3,5 639) F=30Hz Even field (2,4,6 640) 640) Odd field Even field 16.6ms 16.6ms time 480 pixels by 640 pixels Interlaced camera

15 Progressive scan camera All image systems produce a clear image of the background Jagged edges from motion with interlaced scan Motion blur caused by the lack of resolution in the 2CIF sample 1st frame Only progressive scan makes it possible to identify the driver Exp=33.33ms 2nd frame exp=33.33ms